Lyme disease is a tick-borne disease caused by Borrelia burgdorferi sensu lato (s.l.). The disease is typically characterized by the development of an expanding red rash at the site of the tick bite that may be followed by systemic complications including meningitis, carditis or arthritis. Almost all cases of Lyme disease are caused by one of three genospecies, Borrelia afzelii, Borrelia garinii and Borrelia burgdorferi sensu stricto (s.s.). In Europe, all three species which infect humans are found. However, in North America only a single species, Borrelia burgdorferi sensu stricto, is found. Borrelia burgdorferi is a species of Gram negative bacteria of the spirochete class of the genus Borrelia. Antibiotic treatment of Lyme disease is usually effective but some patients develop a chronic disabling form of the disease involving joints or nervous system, which does not substantially improve even after parenteral antibiotic therapy, thus highlighting the need for a vaccine for high-risk populations.
Outer surface protein A (OspA) is a 31 kDa antigen, expressed by Borrelia burgdorferi s.l. species present in the midgut of Ixodes ticks. OspA has proven to be efficacious in preventing Lyme disease in North America (Steere et al., N. Engl. J. Med. 339: 209-15, 1998; Sigal et al., N. Engl. J. Med. 339:216-22, 1998; erratum in: N. Engl. J. Med. 339:571, 1998). The amino terminus of fully processed OspA is a cysteine residue that is post-translationally modified with three fatty-acyl chains that anchor the protein to the outer surface of the bacterial membrane (Bouchon et al., Anal. Biochem. 246: 52-61, 1997). Lipidation of OspA is reported to stabilize the molecule (Luft, personal communication) and is essential for protection in the absence of a strong adjuvant (Erdile et al., Infect. Immun. 61: 81-90, 1993). A soluble, recombinant form of the protein lacking the amino-terminal lipid membrane anchor was co-crystallized with the Fab fragment of an agglutinating mouse monoclonal antibody to determine the structure of OspA, which was shown to comprise 21 anti-parallel β-strands followed by a single α-helix (Li et al., Proc. Natl. Acad. Sci. U.S.A. 94:3584-9, 1997).
A monovalent OspA-based vaccine (LYMErix®) was marketed in the USA for the prevention of Lyme disease. However, in Europe heterogeneity in OspA sequences across the three genospecies precludes broad protection with a vaccine based on OspA from a single strain (Gern et al., Vaccine 15:1551-7, 1997). Seven principal OspA serotypes have been recognized among European isolates (designated serotypes 1 to 7, Wilske et al., J. Clin. Microbiol. 31:340-50, 1993). OspA serotypes correlate with species; serotype 1 corresponds to B. burgdorferi s.s., serotype 2 corresponds to B. afzelii and serotypes 3 to 7 correspond to B. garinii. 
Protective immunity acquired through immunization with OspA is unusual since the interaction between the host's immune response and the pathogen does not take place in the host, but in the mid-gut of the tick vector. In the case of Lyme disease, a tick acts as a vector or carrier for the transmission of Lyme disease from animals to humans. OspA specific antibody acquired during feeding by an infected tick prevents transmission of B. burgdorferi s.l. to the immunized mammalian host (de Silva et al., J. Exp. Med. 183: 271-5, 1996). Protection is antibody-mediated and is mainly affected through bactericidal antibody although an antibody that blocks attachment of the spirochete to a receptor on the lining of the tick gut epithelium may also be efficacious (Pal et al., J. Immunol. 166: 7398-403, 2001).
Rational development of effective OspA vaccines requires identification of the protective epitopes such as that defined by the protective monoclonal antibody LA-2 (Golde et al., Infect. Immun. 65: 882-9, 1997). X-ray crystallography and NMR analysis have been used to identify immunologically important hypervariable domains in OspA and have mapped the LA-2 epitope to amino acids 203-257 (Ding et al., J. Mol. Biol. 302: 1153-64, 2000; Luft et al. J Infect Dis. 185 (Suppl. 1): S46-51, 2002).
There is a need in the art for the development of an OspA vaccine that can provide broad protection against a variety of species of Borrelia that are present in the United States, Europe, and elsewhere. The following disclosure describes the specifics of such a vaccine.